Optical disc drive

ABSTRACT

An optical disc drive using either one of a first disc (e.g., DVD) and a second disc (e.g., CD), is provided with a first laser diode that emits a shorter wavelength beam, a second laser diode that emits a longer wavelength beam, an objective lens, and a driving unit that holds and rotates the optical disc. The optical axis of the objective lens is inclined relative to a normal to the optical disc. The first laser diode is located at a first position so that the coma, which is caused when the first laser beam is converged on a data recording surface of the first disc, is minimized, and the second laser diode is located at a second position so that the coma, which is caused when the second laser beam is converged on a data recording surface of the second disc, is minimized.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to an optical disc drive capable ofrecording and/or reproducing data to/from various types of optical discshaving different characteristics, such as a thickness of a protectivelayer and a data recording density.

[0002] There are a plurality of standards regarding the characteristicsof the optical discs, including the thickness of the protective layerwhich covers a data recording surface of the optical disc and/or thedata recording density. For example, the thickness of the protectivelayer of a CD (Compact Disc) or a CD-R (CD recordable) whose recordingdensity is relatively low is 1.2 mm, while that of a DVD (DigitalVersatile Disc) whose recording density is relatively high is 0.60 mm.

[0003] For recording and/or reproducing data to/from the DVD, since ithas a relatively high data recording density, in order to make the sizeof a beam spot sufficiently small, a laser beam whose wavelength is in arange of approximately 635-660 nm is to be used. For the CD-R, in viewof its reflection characteristics, a laser beam whose wavelength isapproximately 780 nm is to be used.

[0004] The above-described optical discs may preferably be used in asingle optical disc drive. In order to allow a single optical disc driveto use both the DVD and the CD-R, the disc drive is required to have atleast two laser sources respectively emitting the laser beams asdescribed above.

[0005] In the meantime, in view of downsizing of the disc drive, it ispreferable that the optical system adopted in an optical head for thedisc drive is as compact as possible. In particular, optical elementssuch as an objective lens is preferably used for both laser beams, whichenables the laser sources (i.e., laser diodes) to be implemented in asingle package and provided as a light source module. An example of suchan optical disc drive is described in Japanese Patent ProvisionalPublication No. HEI 10-261240.

[0006] If the two laser diodes are incorporated in a single package,beam emitting points of the two laser diodes are aligned in a directionperpendicular to an optical axis of the objective lens. Therefore, atleast one of the beam emitting points is located off the optical axis.

[0007] According to an embodiment described in the above-describedpublication, a semiconductor laser chip (wavelength: 660 nm) for a DVD,whose allowable aberration is relatively small, is positioned on anoptical axis of a lens system including an objective lens, a collimatinglens and the like. The other semiconductor laser chip (wavelength: 780nm) for the CD is located off the optical axis of the lens system.Therefore, the optical disc drive described in the publication has adisadvantage such that aberration, in particular coma, is relativelylarge for the CD (or CD-R).

[0008] Another example of the optical disc drive is disclosed inJapanese Patent Provisional Publication No. HEI 10-261241. The opticaldisc drive disclosed in this publication includes the optical systemsimilar to that disclosed in the aforementioned publication (i.e., HEI10-261240), and further, a holographic optical element (HOE) is added.With this element (HOE), an optical path of the laser beam, whose beamemitting point is located off the optical axis, is deflected (bent) sothat the beam is incident on an objective lens in a direction parallelto the optical axis, thereby suppressing the coma.

[0009] The HOE disclosed in the latter publication (i.e., HEI 10-261241)utilizes a zero order component of the beam for the DVD and a −1st orderdiffraction component of the beam for the CD. Therefore, it is difficultto exhibit high diffraction efficiency for both wavelengths, and loss oflight is relatively large. Further, since an additional element (i.e.,HOE) having a special function as above is added, the manufacturing costof the optical disc drive increases.

SUMMARY OF THE INVENTION

[0010] It is therefore an object of the invention to provide an improvedoptical disc drive that utilizes a light source module including atleast two laser diodes emitting laser beams having differentwavelengths, and a single objective optical system for both wavelengths.In this improved optical disc drive, aberration can be well suppressedwith allowing recording and reproducing of data to/from a plurality oftypes of optical discs to be performed, without adding a special opticalelement such as the HOE.

[0011] For the above object, according to the invention, there isprovided an optical disc drive capable of recording/reproducing datato/from an optical disc, the optical disc being either one of a firstdisc and a second disc, a protective layer of the first disc beingthinner that that of the second disc, the optical disc drive including afirst laser diode that emits a first laser beam having a firstwavelength, a second laser diode that emits a second laser beam having asecond wavelength, the second wavelength being longer than the firstwavelength, an objective lens that converges the first laser beam on thefirst disc, and the second laser beam on the second disc, and a drivingunit that holds and rotates the optical disc. In the optical disc driveconstructed as above, an optical axis of the objective lens is inclinedrelative to a normal to the optical disc, a beam emitting point of thefirst laser diode is located at a first position, coma, which is causedwhen the first laser beam is converged on a data recording surface ofthe first disc, is minimized when the first laser beam is emitted fromthe first position. Further, a beam emitting point of the second laserdiode is located at a second position which is different from the firstposition, and coma, which is caused when the second laser beam isconverged on a data recording surface of the second disc, is minimizedwhen the second laser beam is emitted from the second position.

[0012] With this configuration, whichever laser diode emits the laserbeam, the coma is well suppressed, without employing an additionaloptical element such as HOE.

[0013] Optionally, the objective lens is configured such that coma isminimized for a hypothetical disc under a hypothetical condition, wherethe hypothetical disc is defined as an optical disc having a protectivelayer whose thickness is intermediate between that of the first disc andthat of the second disc, and the hypothetical condition is defined as acondition where the optical axis of the objective lens coincides withthe normal to the optical disc.

[0014] In this case, a first region may be defined on the objectivelens, the first region providing a numerical aperture appropriate forconverging the second laser beam on the second disc, and the objectivelens preferably satisfies the following condition under the hypotheticalcondition where the optical axis of the objective lens coincides withthe normal to the optical disc:

−4.0<SC ₁ /SC ₂<−0.25,

[0015] where, SC₁ represents an offence SC against sine condition at theperipheral portion of the first region when the first laser beam isconverged on the first disc,

[0016] SC₂ represents an offence SC against sine condition at theperipheral portion of the first region when the second laser beam isconverged on the second disc.

[0017] Further, the offence SC against the sine condition is defined bythe formula below:

SC=nH ₁/(n′ sinU′)−f(1−m)

[0018] where, n represents a refractive index on the beam incident sidemedium,

[0019] n′ represents a refractive index on the beam emerging sidemedium,

[0020] U′ represents an angle of the emerging beam with respect to theoptical axis,

[0021] m represents a paraxial magnification,

[0022] H₁ represents a ray height on a principal plane, and

[0023] f represents a focal length.

[0024] Still optionally, the optical axis of the objective lens and thenormal to the optical disc being included in a reference plane, thefirst position and the second position being located on opposite sidewith respect to a reference axis, and the reference axis is an opticalaxis of the objective lens under a hypothetical condition where theoptical axis of the objective lens and the normal to the optical disccoincide with each other. Further, the reference plane is a planeincluding the reference axis and the first and second positions.

[0025] In this case, the first position and the second position may bearranged such that, by arranging the optical axis of the objective lensto be inclined with respect to the normal to the optical disc, the firstlaser beam is converged on a side where a distance between the objectivelens and the optical disc increases, and the second laser beam isconverged on a side where a distance between the objective lens and theoptical disc decreases.

[0026] Furthermore, the objective lens is configured such that coma isminimized for a hypothetical disc under a hypothetical condition, thehypothetical disc being an optical disc having a protective layer whosethickness is intermediate between that of the first disc and that of thesecond disc, the hypothetical condition being a condition where theoptical axis of the objective lens coincides with the normal to theoptical disc.

[0027] Further optionally, the optical disc drive preferably includes afine movement mechanism for driving the objective lens to move forfocusing and tracking, the reference axis being inclined with respect tothe normal to the optical disc, the objective lens being held by thefine movement mechanism such that the optical axis coincides with thereference axis.

[0028] In this case, the driving unit holds the optical disc such thatthe data recording surface of the optical disc extends in parallel witha bottom surface of a case of the optical disc drive.

[0029] In another case, the driving unit may hold the optical disc byinclining the data recording surface with respect to a bottom surface ofa case of the optical disc drive so that the reference axis isperpendicular to a bottom surface of a case of the optical disc drive.

[0030] Alternatively, driving unit may hold the optical disc such thatthe data recording surface of the optical disc extends in parallel witha bottom surface of a case of the optical disc drive.

DESCRIPTION OF THE ACCOMPANYING DRAWINGS

[0031]FIG. 1 is a schematic representation of an optical disc driveaccording to a first embodiment of the invention;

[0032]FIGS. 2A and 2B show a structure of the lens actuator 12: FIG. 2Ashows a plan view which is viewed from the optical disc 2; and FIG. 2Bis a side view;

[0033]FIG. 3 schematically shows a front view of the polarizingdiffractive grating of the composite optical element;

[0034] FIGS. 4A-4C show a structure of the laser module 9:

[0035]FIG. 4A is a front view;

[0036]FIG. 4B is a cross-sectional view taken along line B-B′ of FIG.4A; and

[0037]FIG. 4C is an enlarged view of a portion of FIG. 4B;

[0038]FIG. 5 is an enlarged view of the semiconductor base plate;

[0039]FIG. 6 shows a block diagram of a signal processing circuit;

[0040]FIG. 7 illustrates compensation for coma;

[0041]FIG. 8 is a graph showing a relationship between the incidentangle and quantity of coma when the DVD and CD (or CD-R) are used;

[0042]FIG. 9 is a developed view of the optical system shown in FIG. 1for illustrating optical paths;

[0043]FIG. 10 shows a schematic representation of an optical disc driveaccording to a second embodiment of the invention;

[0044]FIGS. 11A and 11B show the structure of the lens actuator employedin the optical disc drive shown in FIG. 10:

[0045]FIG. 11A is a plan view viewed from the optical disc side; and

[0046]FIG. 11B is a cross-sectional side view;

[0047]FIG. 12 shows a schematic representation of an optical disc driveaccording to a third embodiment of the invention;

[0048] FIGS. 13A-13C show the structure of the objective lens;

[0049]FIG. 14 schematically shows the objective lens and the DVD;

[0050]FIG. 15A shows spherical aberration SA and deviation SC of sinecondition at the wavelength of 659 nm;

[0051]FIG. 15B shows chromatic aberration represented by sphericalaberration for wavelengths of 654 nm, 659nm and 664 nm;

[0052]FIG. 15C shows astigmatism (DS: sagittal; and DM: meridional);

[0053] FIGS. 16A-16H show wavefront aberration, under the hypotheticalcondition shown in FIG. 14, when the beam emitted by the first laserdiode is incident on the objective lens at a predetermined incidentangle;

[0054]FIG. 17 is a graph showing a relationship between the incidentangle of the beam emitted from the first laser diode with respect to theobjective lens and wavefront aberration when the DVD is used;

[0055]FIG. 18 schematically shows the objective lens and the secondoptical disc under the hypothetical condition;

[0056] FIGS. 19A-19C show aberrations of the objective lens under thehypothetical condition shown in FIG. 18 when the laser beam emitted bythe second laser diode is incident on the objective lens at the incidentangle of 0 degree;

[0057] FIGS. 20A-20H show wavefront aberration under the hypotheticalcondition shown in FIG. 18, when the beam emitted by the second laserdiode is incident on the objective lens at a predetermined incidentangle;

[0058]FIG. 21 is a graph showing a relationship between the incidentangle of the beam emitted from the second laser diode with respect tothe objective lens and wavefront aberration (rms: root-mean-squarerepresentation);

[0059]FIG. 22 shows a relationship between the wavefront aberration (rmsrepresentation) and the angle formed between the first laser beamincident on the objective lens and the reference axis, when the DVD isused, and the normal to the optical axis is inclined at 0.24 degreeswith respect to the reference axis;

[0060]FIG. 23 shows a relationship between the wavefront aberration (rmsrepresentation) and the angle formed between the second laser beamincident on the objective lens and the reference axis when the CD isused, and the normal to the optical axis is inclined at 0.24 degreeswith respect to the reference axis;

[0061]FIG. 24 shows a relationship between the wavefront aberration (rmsrepresentation) and the angle formed between the first laser beamincident on the objective lens and the reference axis when the DVD isused, and the optical axis of the objective lens is inclined at −0.15degrees with respect to the reference axis; and

[0062]FIG. 25 shows a relationship between the wavefront aberration (rmsrepresentation) and the angle formed between the second laser beamincident on the objective lens and the reference axis when the CD isused, and the optical axis of the objective lens is inclined at −0.15degrees with respect to the reference axis.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0063] Referring to the accompanying drawings, embodiments of theinvention will be described.

[0064] Firstly, three optical disc drives capable ofrecording/reproducing data to/from DVD, CD and CD-R discs according tothe embodiments will be described. Thereafter, concrete examples of anobjective lens used for the optical disc drives will be described.

[0065] First Embodiment

[0066]FIG. 1 schematically shows an optical disc drive 1 according to afirst embodiment of the invention.

[0067] The optical disc drive 1 is provided with a motor 3 for rotatingan optical disc 2, and an optical head 6 accommodating a laser module 9and an objective lens 16. The motor 3 is mounted on a bottom surface ofa casing (not numbered) of the optical disc drive 1 via a mounting base4. The optical disc 2 is mounted to a spindle shaft 5 of the motor 3such that the data recording surface thereof faces, in parallel with,the bottom surface of the optical disc drive 1. The optical disc 2 canbe either a first disc, e.g., a DVD or a second disc, e.g., a CD or aCD-R. The first disc has a protective layer which is 0.6 mm thick, andthe wavelength of the laser beam for recording/reproducing data to/fromthe first disc is approximately 659 nm. The second disc has a protectivelayer which is 1.2 mm thick, and the wavelength of the laser beam forrecording/reproducing data to/from the second disc is approximately 790nm.

[0068] The optical head 6 is driven by a rough moving mechanism 7including a voice coil motor on a rail member 8 for seeking, in atracking direction (indicated by arrow T). In the laser module 9included in the optical head 6, a first laser diode 14 a which emits ashorter wavelength (659 nm) laser beam 13 a and a second laser diode 14b which emits a longer wavelength (790 nm) laser beam 13 b. The firstlaser diode 14 a and the second laser diode 14 b are arranged such thatthe light emitting points thereof are located close to each other.

[0069] The optical head 6 further includes a collimating lens 10 whichcollimates the diverging beams emitted by the first and second laserdiodes 14 a and 14 b, respectively, and a mirror 11 for deflecting thelaser beams, which are emitted by the first and second laser diodes 14 aand 14 b, collimated by the collimating lens 10, and proceed in adirection parallel to the data recording surface of the optical disc 2,to direct to impinge on the objective lens 16. The objective lens 16 issecured to a lens actuator 12 together with a composite element 15,which will be described later.

[0070] The lens actuator 12 is a fine movement mechanism, which drivesthe objective lens 16 and the composite element 15 integrally in adirection normal to the data recording surface of the optical disc 2 andin a direction of a radius of the optical disc 2 for focusing andtracking.

[0071] The objective lens 16 converges the laser beam 13 a (approx. 659nm) emitted by the first laser diode 14 a on the data recording surfaceof a DVD whose protective layer is relatively thin (0.6 mm), andconverges the laser beam 13 b (approx. 790 nm) emitted by the secondlaser diode 14 b on the data recording surface of a CD or CD-R whoseprotective layer is relatively thick (1.2 mm).

[0072] The optical axis 16X of the objective lens 16 is inclined withrespect to the normal to the optical disc 2. For the sake ofdescription, a reference axis 10X and a reference plane are defined. Thereference axis 10X is a hypothetical optical axis of the objective lens16 under a hypothetical condition where the optical axis 16X coincideswith the normal to the optical axis. Further, the reference plane is aplane including the reference axis 10X and the beam emitting pints ofthe first and second laser diodes 14 a and 14 b. The reference axis 10Xis, in other words, an optical axis of the collimating lens 10, and thereference plane is a plane parallel to the surface of FIG. 1.

[0073] In the first embodiment, the normal to the optical disc 2 isincluded in the reference plane and inclined with respect to thereference axis 10X. Further, the optical axis 16X of the objective lens16 coincides with the reference axis 10X. The reference axis 10X, whichcoincides with the optical axis of the collimating lens 10, is deflectedby a reflection surface 11R of the mirror 11 at an angle, which isformed between the axes 10X before and after deflected, greater than 90degrees. The deflected axis is inclined with respect to the normal tothe optical disc 2. The objective lens 16 is configured such that, underthe hypothetical condition, i.e., if the optical axis 16X of theobjective lens 16 is normal to the optical disc 2, the coma is minimizedfor a hypothetical disc that is an optical disc having a protectivelayer whose thickness is intermediate between 0.6 mm and 1.2 mm.

[0074] With this condition, the sign of the coma is opposite between acase where the DVD is mounted and a case where the CD or CD-R is mountedin the above-defined hypothetical condition. IF the normal to theoptical disc is inclined with respect to the optical axis of theobjective lens, a first position of the beam emitting point of the laserbeam 13 a and a second position of the beam emitting point of the laserbeam 13 b are determined. The first position is defined such that if thebeam emitting point of the lser beam 13 a is located at the firstposition, the coma is minimized when the laser beam 13 a is converged onthe data recording surface of the DVD. The second position is definedsuch that if the beam emitting point of the laser beam 13 b is locatedat the second position, the coma is minimized when the laser beam 13 bis converged on the data recording surface of the CD or CD-R. The fistposition and the second position are located on opposite sides withrespect to the reference axis 10X. Therefore, the first and second laserdiodes 14 a and 14 b are located on opposite sides with respect to thereference axis 10X. An incident angle at which the laser beam 13 a isincident on the optical disc 2 is greater than an incident angle atwhich the laser beam 13 b is incident on the optical disc 2.

[0075] With the above configuration, i.e., by inclining the normal tothe optical disc 2 with respect to the optical axis 16X of the objectivelens 16, the first laser beam 13 a is converged on a side, with respectto the reference axis 10X, where a distance between the objective lens16 and the optical disc 2 increases, and the second laser beam 13 b isconverged on a side, with respect to the reference axis 10X, where adistance between the objective lens 16 and the optical disc 2 decreases.

[0076]FIGS. 2A and 2B show a structure of the lens actuator 12: FIG. 2Ashows a plan view which is viewed from the optical disc 2; and FIG. 2Bis a side view. The objective lens 16 and the composite element 15 areintegrally held by a lens holder 31. The lens holder 31 is secured ontoa holding base 33 using wire spring 32. On both side surfaces of thelens holder 31, coils 34 are provided, while on both sides of the lensholder 31, at positions facing the coils 34, magnets 35 are fixedlyprovided. The magnets 35 and the holding base 33 are secured to thecasing 37 of the optical head. By supplying electrical current to thecoils 34, the lens holder 31 can be moved, due to electromagnetic forcegenerated between the coils 34 and magnets 35, in a direction parallelto the normal to the optical disc 2 (i.e., in a focusing directionindicated by arrow F), thereby focusing being performed, and in a radialdirection of the optical disc 2 (i.e., in a tracking direction indicatedby arrow T), thereby tracking being performed. It should be noted that,by the quantity of electromagnetic force between the coils 34 and themagnets 35, the moving amount of the lens holder 31 in the focusingdirection F is adjusted, and by the balance between the forces generatedbetween each pair of coil 34 and magnet 35, the moving amount of thelens holder 31 in the tracking direction T is adjusted.

[0077] The composite element 15 is an element having a {fraction (1/4)}wavelength plate and a polarizing diffractive grating. The polarizingdiffractive grating is configured as shown in FIG. 3. That is, an areaof the diffractive grating is divided into four sub-areas by twoboundaries 41 and 42 crossing at right angles. A circled portion 43indicated in FIG. 3 represent an area on which the laser beam 13 a isincident. The incident beam is emerged from the polarizing diffractiongrating. The ordinary rays are emerged from the polarizing grating asthey are, while the extraordinary rays are emerged from each of thesub-areas as +1st and −1st order diffraction components. Grid patternsof the diffraction grating structures at the four sub-areas aredifferent, but pitches are identical. Therefore, the eight diffractioncomponents emerged from the diffraction grating are diffracted indifferent directions but the diffraction angles thereof are the same.The eight diffraction components are collimated by the collimating lens10 and converged inside the laser module 9 as eight beam spots.

[0078] FIGS. 4A-4C show a structure of the laser module 9: FIG. 4A is afront view; FIG. 4B is a cross-sectional view taken along line B-B′ ofFIG. 4A; and FIG. 4C is an enlarged view of a portion of FIG. 4B.

[0079] The laser module 9 Includes a package 21 formed of heatconductive material such as aluminum nitride, and a plurality of leadlines 22 penetrated through the package 21 to transmits electricalsignals. Inside the package 21, a semiconductor base plate 24 made ofsilicon or the like is formed. The upper surface of the package 21 iscovered with a glass plate 23 which transmits light. On thesemiconductor base plate 24, a recessed portion 25 is formed. Betweenthe upper surface and bottom surface of the recessed portion 25, aninclined surface 26 is formed (see partially enlarged view of FIG. 4C).The inclined surface 26 is formed to be a mirror surface, which inclineswith respect to the bottom surface at 45 degrees. The laser diodes 14 aand 14 b are mounted on the bottom surface of the recessed portion 25such that beam emitting points thereof face the mirror surface 26. Thelaser beams 13 a and 13 b are emitted toward the mirror surface 26(i.e., in the right-hand side in FIG. 4B), which are reflected by themirror surface 26 and directed upward in the figure through the glassplate 23.

[0080]FIG. 5 is an enlarged view of the semiconductor base plate 24. Onthe semiconductor base plate 24, four pairs of photo detectors 53 a and53 b for obtaining focusing error signals, and four photo detectors 56for obtaining tracking error signals and data reproducing signal areformed. The photo detectors 53 a and 53 b are connected as shown in FIG.5 using conductive thin layer 54 made of aluminum or the like, andfurther connected to terminals A and B of a pat for bonding wires. Thefour photo detectors 56 are connected to terminals C, D, E and F,respectively.

[0081] Each pair of the photo detectors 53 a and 53 b are formed as apair of rectangular light receiving areas aligned in a direction of theshorter side of the rectangle, since the focusing error is detected inaccordance with the double knife edge method. When the beam spots arefocused on the data recording surface of the optical disc, thequarter-circular beam spots are focused on the pairs of rectangularareas, respectively. If the focal plane of the beam spots is away fromthe data recording surface, the size of the beam spots on the detectorsincreases: if the focal plane is located in front of the data recordingsurface, the beam spots on each pair of detecting areas shift in onedetecting area side; and if the focal plane is located on a rear side ofthe data recording surface, the beam spots on each pair of detectingareas shift in the other detecting area side. By calculating adifference between (a) a sum of outputs of detecting areas which receivegreater amount of light when the focal plane is in front of the datarecording surface, and (b) a sum of outputs of detecting areas whichreceive greater amount of light when the focal plane is located on therear side of the data recording surface, the focusing error signal isobtained. Since the focusing error signal is obtained based on outputsof a plurality of pairs of detecting areas, effects of the positionalerrors of beam spots can be cancelled. Therefore, a relatively highaccuracy of the focusing error signal can be maintained without preciseadjustment of the positions of the detectors.

[0082] The laser beams 13 a and 13 b emitted by the laser diodes 14 aand 14 b are reflected by the mirror 26 at points 52 a and 52 b,respectively, and directed in a direction perpendicular to the surfaceof FIG. 5. The eight filled-in quarter-circles 51 a indicate beam spotsformed by beams divided by the composite element 15 into eight afteremitted by the laser diode 14 a and reflected by the optical disc 2(DVD). The filled-in quarter-circles 51 a are aligned on a circlecentered about the position 52 a. The eight hollow quarter-circles 51 bindicate beam spots formed by beams divided by the composite element 15into eight after emitted by the laser diode 14 b and reflected by theoptical disc 2 (CD, CD-R). The hollow quarter-circles 51 b are alignedon a circle centered about the position 52 b. Among eight beam spotscorresponding to each of the laser beams 13 a and 13 b, spots formed byfour −1st order components are converged on the pairs of the photodetectors 53 a and 53 b, respectively, while the remaining four +1storder components are converged on the four photo detectors 56,respectively.

[0083] The signals from terminals A-F are processed by a signalprocessing circuit shown in FIG. 6.

[0084] A differential amplifier 61 receives signals from terminals A andB, and outputs a difference between the received signals. The outputsignal represents a focusing error signal 62, which is obtained inaccordance with a knife-edge method.

[0085] A differential amplifier 63 a receives the output of an adder 63b, which adds signals from the terminals C and D, and the output of anadder 63 c, which add signals from the terminals E and F. The output ofthe differential amplifier 63 a represents a tracking error signal 64,which is obtained in accordance with a push-pull method when the opticaldisc has guide grooves. The adder 63 d adds the output signals of theadders 63 b and 63 c to output data reproducing signal 65. Adifferential amplifier 66 a receives the output of an adder 66 b whichadds the signals from terminals C and E, and the output of an adder 66 cwhich adds the signals from the terminals D and F. The output of thedifferential amplifier 66 a represents a tracking error signal 67 whichis obtained in accordance with a DPD (differential phase detection)method when the optical disc is provided with guiding pit.

[0086] By supplying focusing error signal and the tracking error signaldetected as described above to the coils 34 of the lens actuator 12, theobjective lens 16 is moved in the direction of the optical axis and/orin the radial direction of the optical disc 2, thereby focusing andtracking being achieved.

[0087] When data is recorded on the DVD or CD-R, the intensity of thelaser beam emitted by the laser diode 14 a or 14 b is modulated inaccordance with the data to be recorded. When the data recorded on theoptical disc 2 is reproduced, the intensity of the laser beam emitted bythe first laser diode 14 a or the second laser diode 14 b is maintainedat a predetermined constant level, and the data is reproduced based onthe data reproducing signal 65, which is detected as described above.

[0088]FIG. 7 is a drawing for illustrating compensation of a coma.

[0089] Firstly, it is assumed that there is a disc 71 (which is ahypothetical disc) having a protective layer, whose thickness is betweenthe thickness of the protective layer of the CD (i.e., 0.6 mm) and thatof the DVD (i.e., 1.2 mm), and the normal to the hypothetical disc 71coincides with the optical axis 16X of the objective lens 16. Further,it is assumed that a beam is incident on the objective lens 16 having anangle θ with respect to the optical axis 16X, and 74 denotes a principalray of the beam, and 75 and 76 denote rim rays. Points A and B representpoints at which the rays 75 and 76 intersect with the beam incident sidesurface of the hypothetical disc 71, respectively.

[0090] If the coma of the objective lens, with respect to the disc 61,is compensated for, the beam which is incident on the objective lens 16with a certain incident angle, the beam is converged at point S throughthe protective layer. Such an objective lens can be configured byforming an aspherical surface and/or combining a plurality of sphericallenses.

[0091] If a disc 71 is replaced with a CD or CD-R having a relativelythick protective layer, a position of the beam incident side surface isshifted from 72 to 72′, thereby aberration being caused. For the sake ofdescription, the points where the rays 75 and 76 intersect the surface72′ are represented by A′ and B′.

[0092] If the incident angle θ of the beam incident on the objectivelens 16 is zero, the aberration generated is mainly spherical aberrationwhich is symmetrical with respect to the principal ray 74. In such acase, the aberration can be compensated for by changing degree ofdivergence or convergence of the incident beam, or using diffractivegrating having annular zones. However, if the incident angle θ is notzero, as shown in FIG. 7, the increase of the optical path betweenpoints A′-A is greater than the increase of the optical path betweenpoints B′-B, and therefore coma is caused. Assuming that the sign of thecoma in this condition is negative, then, if the incident beam inclines,with respect to the optical axis 16X, in a direction opposite to thatshown in FIG. 7, the sign of the coma is positive.

[0093] If the disc 71 is replaced with a DVD that has a thinnerprotective layer, the position of the beam incident side surface 72 ischanged to 72″, thereby aberration being caused. It is assumed that thepoints where the rays 75 and 76 intersect the surface 72″are A″ and B″.Then, if the incident angle θ of the beam incident on the objective lens16 is not zero, as shown in FIG. 7, the reduction of the optical pathlength within A-A″ is greater than that in B-B″, thereby coma beingcaused. The sign of this coma is positive. If the inclination of thebeam is opposite with respect to the optical axis 16X, then the coma isnegative.

[0094]FIG. 8 is a graph showing a relationship between the incidentangle θ and quantity of coma with the sign thereof when the DVD and CD(or CD-R) are used instead of the hypothetical disc 71. The horizontalaxis represents the incident angle θ and the vertical axis representsthe quantity of coma. In the graph shown in FIG. 8, a curve 81 indicatedby solid line represents the characteristic when the DVD replaces thedisc 71, and a curve 82 indicated by broken lines represents thecharacteristic when the CD (or CD-R) replaces the disc 71.

[0095] For example, if the incident angle θ is positive when the DVD isused, positive coma is caused. If the incident angle θ is negative whenthe CD is used, positive coma is caused, too. It should be noted thatthe quantity of coma for a predetermined incident angle can be varied bychanging the shape of the objective lens 16. For example, if theobjective lens 16 is configured to compensate for the coma caused by theoptical disc 71 having a relatively thick protective layer, theinclination of the curve 81 will be steeper, and the inclination of thecurve 82 will be gentler than those shown in FIG. 8. On the contrary, ifthe protection layer of the disc 71 is assumed to be thinner, and theobjective lens 16 is designed to compensate for the coma caused by thethus modified optical disc 71, the inclination of the curve 81 isgentler and the inclination of the curve 82 will be steeper than thoseshown in FIG. 8.

[0096]FIG. 9 is a developed view of the optical system shown in FIG. 1,illustrating the optical paths. The reference axis 10X and the opticalaxis 16X of the objective lens 16 coincide with each other. Further, thelaser diodes 14 a and 14 b are arranged on opposite sides with respectto the reference axis 10X. With this configuration, the laser beam 13 aemitted by the first laser diode 14 a and the laser beam 13 b emitted bythe second laser diode 14 b are both collimated by the collimating lens10, and incident on the objective lens 16 with being inclined inopposite directions with respect to the reference axis 10X. Therefore,for each of the laser beams 13 a and 13 b, positive coma is caused.

[0097] Further, the DVD 91 and the CD 92 are inclined, with respect tothe reference axis 10X, in the same direction. With this inclinedarrangement, negative coma is caused when either of the DVD 91 and theCD 92 is used. Therefore, the positive coma caused by the inclination ofthe beams with respect to the reference axis 10X can be cancelled by theinclined arrangement of the optical discs.

[0098] Between the DVD and the CD, the thickness of the protectivelayers and the numerical apertures are different. Therefore, withrespect to the same incident angle θ, the quantity of the coma causedthereby are different. However, as afore-mentioned, the quantity of comacan be adjusted by changing the shape of the objective lens 16, andtherefore, the coma can be cancelled for both the DVD and CD.

[0099] Second Embodiment

[0100]FIG. 10 shows an optical disc drive 101 according to a secondembodiment of the invention, and FIGS. 11A and 11B show a lens actuator104 employed in the optical disc drive 101.

[0101] The basic structure of the optical disc drive 101 is similar tothat of the optical disc drive 1 according to the first embodiment. Theoptical disc 2 is secured to a spindle shaft 5 of a motor 3, which ismounted on a mounting base 4. An optical head 102 is movable long therail member 8.

[0102] Inside the optical head 102, a laser module 9, a collimating lens10, a mirror 103, a lens actuator 104 are incorporated. The lensactuator 104 holds the objective lens 16 and the composite element 15.The laser module 9 and the collimating lens 10 are similar to thoseemployed in the optical disc drive 1 according to the first embodiment.

[0103] In the second embodiment, the optical axis 16X of the objectivelens is inclined with respect to the reference axis 10X in the referenceplane, and the normal to the optical disc 2 coincides with the referenceaxis 10X. The reference axis 10X is bent by 90 degrees on the refectionsurface 103R of the mirror 103, and extends in a direction parallel tothe normal to the optical disc 2. The objective lens 16 is configuredsuch that, the coma is minimized for a hypothetical disc whoseprotective layer has an intermediate thickness between 0.6 mm and 1.2 mmin a hypothetical condition where the optical axis 16X coincides withthe normal to the optical disc 2.

[0104] With the above configuration, in the hypothetical condition wherethe optical axis 16X coincides with the normal to the optical disc 2,the sign of the coma when the DVD is used and when the CD (or the CD-R)is used is different. In this condition, if the optical axis 16X isinclined with respect to the reference axis 10X, a first position of thebeam emitting point of the laser beam 13 a and a second position of thebeam emitting point of the laser beam 13 b are determined. The firstposition is defined such that if the beam emitting point of the laserbeam 13 a is located at the first position, the coma is minimized whenthe laser beam 13 a is converged on the data recording surface of theDVD. The second position is defined such that if the beam emitting pointof the laser beam 13 b is located at the second position, the coma isminimized when the laser beam 13 b is converged on the data recordingsurface of the CD or CD-R. The fist position and the second position arelocated on opposite sides with respect to the reference axis 10X.Therefore, the first and second laser diodes 14 a and 14 b are locatedon opposite side with respect to the reference axis 10X.

[0105] The optical axis 16X is inclined such that the incident angle ofthe laser beam 13 a with respect to the objective lens 16 is smallerthan the incident angle of the laser beam 13 b with respect to theobjective lens 16. With this configuration, i.e., by inclining theoptical axis 16X of the objective lens 16 relative to the normal to theoptical disc 2, the first laser beam 13 a is converged on a side wherethe distance between the objective lens and the optical disc increases,and the second laser beam 13 b is converged on a side where the distancebetween the objective lens 16 and the optical disc 2 decreases. Withthis arrangement, the coma which is caused since the laser beams areincident on the objective lens 16 with being inclined with respect tothe reference axis 10X can be cancelled.

[0106]FIGS. 11A and 11B show the structure of the lens actuator 104:FIG. 11A is a plan view viewed from the optical disc side; and FIG. 11Bis a cross-sectional side view.

[0107] Similarly to the first embodiment, the objective lens 16 and thecomposite element 15 are integrally held by the lens holder 111, whichis mounted by wire springs 32 onto the holding base 33. The objectivelens 16 is inclined with respect to the reference axis 10X, while thecomposite element 15 is not inclined. The structure and arrangement ofthe coils 34 and the magnets 35 are similar to those in the firstembodiment.

[0108] Third Embodiment

[0109]FIG. 12 schematically shows a configuration of an optical discdrive 121 according to a third embodiment of the invention.

[0110] The optical disc 5 is secured to the spindle shaft 5 of the motor3, which is mounted on the mounting base 122. An optical head 123 isdriven to move on a rail member 126 by a rough movement mechanism 125.

[0111] In the optical head 123, a laser module 9, a collimating lens 10,a mirror 103, a lens actuator 124 are provided. The lens actuator 124holds an objective lens 16 and a composite element 15. The laser module9, the collimating lens 10 and the mirror 103 function similarly tothose described with reference to FIG. 10. It should be noted, however,in the third embodiment, the arrangement of the laser diodes 14 a and 14b are opposite to that in the other embodiments.

[0112] According to the third embodiment, the normal to the optical disc2 is inclined with respect to the reference axis 10X in the referenceplane. Further, the optical axis 16X of the objective lens 16 coincideswith the reference axis 10X. Furthermore, the optical disc 2 issupported as being inclined with respect to the bottom surface of thecasing of the optical disc drive 121 such that the reference axis 10X isperpendicular to the bottom surface of the casing of the optical discdrive 121. Due to the above arrangement, the surface of the mountingbase 122 on which the motor 3 is mounted is inclined with respect to thebottom surface of the casing of the optical disc drive 121, and the railmember 126 is also inclined so that the optical head 123 is movable in adirection parallel to the data recording surface of the optical disc 2.

[0113] The reference axis 10X, which is inclined with respect to thenormal to the optical axis 2, is bent by the reflection surface 103R ofthe mirror 103 at 90 degrees. The objective lens 16 is configured suchthat, in a hypothetical condition where the optical axis 16X coincideswith the normal to the optical disc 2, the coma is minimized for ahypothetical disc whose protective layer has an intermediate thicknessbetween 0.6 mm and 1.2 mm.

[0114] With the above configuration, in the hypothetical condition wherethe optical axis 16X coincides with the normal to the optical disc 2,the signs of the coma when the DVD is used and when the CD (or CD-R) isused are different.

[0115] If the normal to the optical disc 2 is inclined with respect tothe reference axis 10X, a first position of the beam emitting point, atwhich the coma is minimized when the laser beam 13 a is converged on thedata recording surface of the DVD, and a second position of the beamemitting point, at which the coma is minimized when the laser beam 13 bis converged on the data recording surface of the CD (or CD-R) aredetermined on opposite side with respect to the reference axis 10X, aredetermined. The laser diodes 14 a and 14 b are arranged such that thelight emitting points thereof are located at the first and secondpositions above. Therefore, the laser diodes 14 a and 14 b are locatedon opposite sides with respect to the reference axis 10X.

[0116] The normal to the optical disc 2 is inclined with respect to thereference axis 10X in a direction where the incident angle of the laserbeam 13 a with respect to the optical disc 2 is greater than theincident angle of the laser beam 13 b with respect to the optical disc2. With this configuration, by inclining the optical axis of theobjective lens relative to the normal to the optical disc, the firstlaser beam is converged on a side where a distance between the objectivelens and the optical disc increases, and the second laser beam isconverged on a side where a distance between the objective lens and theoptical disc decreases. By arranging the optical disc 2 to incline, thecoma which is caused as the laser beam is incident on the objective lens16 with being inclined with respect to the reference axis 10X can becancelled.

[0117] Next, the structure of the objective lens 16 will be described indetail.

[0118] FIGS. 13A-13C show the structure of the objective lens 16. FIG.13A is a front view, FIG. 13B is a cross-section taken along the centralline in FIG. 13A, and FIG. 13C is a partially enlarged view of thesurface of the objective lens 16. The objective lens 16 is a single lenselement having two convex aspherical surfaces 16 a and 16 b made ofsynthetic resin. On the surface 16 a, annular zones, which areconcentric with respect to the optical axis of the objective lens 16,are formed as shown in FIG. 13C to provide a diffractive lens structure.As schematically shown in FIG. 13C, at borders between the annularzones, steps extending in parallel with the optical axis are formed asin the Fresnel lens structure.

[0119] The spherical aberration of the optical system of the opticaldisc drive changes toward an overcorrected direction as the thickness ofthe protective layer increases. While, for the DVD which has arelatively thin protective layer, the laser beam having a shorterwavelength is used, and for the CD which has a relatively thickprotective layer, the laser beam having a longer wavelength is used.Therefore, the optical system is configured such that the sphericalaberration is compensated for when the DVD is used and therefore theshorter wavelength beam is used. Further, the diffractive lens structureis given the characteristic such that the spherical aberration changesto undercorrected direction when the wavelength increases. With thisconfiguration, the spherical aberration which changes to theovercorrected direction when the optical disc is changed from DVD to CD(i.e., the thickness of the protective layer increases) can be cancelledby the spherical aberration provided by the diffractive lens structure,which changes in the undercorrected direction when the optical disc ischanged from DVD to CD (i.e., the wavelength of the laser beamincreases).

[0120] The surface of the objective lens 16 can be divided into a commonregion Rc and a high NA region Rh. The common region Rc provides arelatively low NA which is necessary and sufficient for forming a validbeam spot on the optical disc having a relatively low recording density,i.e., the CD, CD-R, and the like. The high NA region Rh providestogether with the common region Rc, a relatively high NA which Isnecessary for forming a valid beam spot on the optical disc having arelatively high recording density, i.e., DVD. The diffractive lensstructure is formed on the whole area of the first surface 16 a,including the common region Rc and the high NA region Rh.

[0121] The objective lens 16 is formed to satisfy the followingcondition (1) under the hypothetical condition where the optical axis ofthe objective lens 16 and the normal to the optical disc 2 coincide witheach other.

−4.0<SC ₁ /SC ₂<−0.25  (1)

[0122] where, SC₁ represents an offense SC against sine condition at theperipheral portion of the common region Rc when the shorter wavelength(e.g., 790 nm) laser beam is converged on the first optical disc (e.g.,CD),

[0123] SC₂ represents an offense SC against sine condition at theperipheral portion of the common region Rc when the longer wavelength(e.g., 659 nm) laser beam is converged on the second optical disc (e.g.,DVD), and

[0124] the offense SC against the sine condition is defined by theformula below.

SC=nH ₁/(n′ sinU′)−f(1−m)

[0125] where, n represents a refractive index on the beam incident sidemedium (i.e. the air),

[0126] n′ represents a refractive index on the beam emerging side medium(i.e., the protective layer),

[0127] U′ represents an angle of the emerging beam with respect to theoptical axis,

[0128] m represents a paraxial magnification,

[0129] H₁ represents a ray height on a principal plane, and

[0130] f represents a focal length.

NUMERICAL EXAMPLE

[0131] Hereinafter, a concrete embodiment in accordance with theabove-described embodiments will be described.

[0132]FIG. 14 schematically shows the objective lens 16 and the DVD 91in the hypothetical condition where the optical axis 16X of theobjective lens 16 coincides with the normal to the DVD 91. Thediffractive lens structure is formed on the surface 16 a, and the firstorder diffractive component is converged on the data recording surfaceof the DVD 91. The surface 16 b is formed as an aspherical surfacewithout steps.

[0133] The numerical structure of the objective lens 16 is indicated inTables 1-3. Table 1 indicates an overall specification of the objectivelens 16. Tables 2 and 3 indicate the data of the first and secondsurfaces 16 a and 16 b, respectively. The common region Rc is a regionfor 0≦h<1.25 (unit: mm), and the high NA region Rh is a region for1.25≦h<1.40 (unit: mm). In Tables, λ₁, NA₁, f₁ represent the wavelength,NA, focal length when the first disc (e.g., DVD) is used, and λ₂, NA₂,f₂ represent the wavelength, NA, focal length when the second disc(e.g., CD) is used. Further, nλ represents the refractive index for thewavelength λ. TABLE 1 λ1 = 659 nm NA1: 0.60 f1 = 2.343 mm λ2 = 790 nmNA2: 0.53 f2 = 2.360 mm distance between 1st and 2nd surfaces 1.400 mmrefractive index n659 = 1.54048 n790 = 1.53654 Abbe number ν: 55.6thickness of protective layer DVD: 0.600 mm CD: 1.200 mm

[0134] The base curves (i.e., the shape of a refractive lens excludingthe diffractive lens structure) and the diffractive lens structures incommon region Rc and in the high NA region Rh of the first surface 16 ahave different shapes and functions.

[0135] The aspherical surface defining the base curve is expressed bythe following equation.${X(h)} = {\frac{{Ch}^{2}}{1 + \sqrt{1 - {\left( {1 + K} \right)C^{2}h^{2}}}} + {A_{4}h^{4}} + {A_{6}h^{6}} + {A_{8}h^{8}} + {A_{10}h^{10}} + {A_{12}h^{12}}}$

[0136] where, h represents a height of a point on the aspherical surfacewith respect to the optical axis,

[0137] X(h) represents a SAG (i.e., a distance of the point from a planetangential to the aspherical surface at the optical axis),

[0138] C represents a curvature (i.e., 1/r, r being a radius ofcurvature),

[0139] K represents a conical coefficient, and

[0140] A₄, A₆, A₈, A₁₀, A₁₂ represent 4th, 6th, 8th 10th, 12th orderaspherical coefficients, respectively.

[0141] Further, an additional optical path length, which is added by thediffractive lens structure, is represented by a optical path differencefunction φ(h) below.

φ(h)=(P ₂ h ² +P ₄ h ⁴ +P ₆ h ⁶+ . . . )×m×λ

[0142] where, h represents a height from the optical axis,

[0143] P_(n) represents an n-th (n being even) order optical pathdifference coefficient,

[0144] m represents the order of diffraction, and

[0145] λ represents a wavelength.

[0146] The optical path difference function φ(h) represents an opticalpath difference between a diffracted ray, at the ray height of h, and aray when the ray would not be diffracted by the diffractive lensstructure.

[0147] In Table 2, the aspherical coefficient, the optical pathdifference function coefficients for the first surface 16 a areindicated. λB represents a blaze wavelength of the diffractive lensstructure. TABLE 2 First surface 16a common region high NA region (0 ≦ h< 1.25) (1.25 ≦ h < 1.40) r  1.498  1.541 κ −0.500 −0.500 A₄ −1.0030 ×10⁻³ −2.3100 × 10⁻³ A₆ −8.9000 × 10⁻⁴  6.0600 × 10⁻⁵ A₈ −2.0960 × 10⁻³−1.0900 × 10⁻⁴ A₁₀  1.1530 × 10⁻³  1.0300 × 10⁻⁴ A₁₂ −4.7260 × 10⁻⁴−2.2500 × 10⁻⁴ P₂  0.0000 −7.6387 P₄ −6.9320 −1.5000 P₆ −1.2190  0.0000P₈  0.0000  0.0000 P₁₀  0.0000  0.0000 λB 720 nm 659 nm

[0148] TABLE 3 Second surface 16b entire region r −5.396 κ  0.0000 A₄−3.4870 × 10⁻² A₆ −4.5550 × 10⁻³ A₈ −8.0120 × 10⁻³ A₁₀  4.1480 × 10⁻³A₁₂ −7.0080 × 10⁻⁴

[0149] FIGS. 15A-15C show, under the hypothetical condition shown inFIG. 14, aberrations of an optical system including the objective lens16 and the optical disc when the laser beam 13 a is incident on theobjective lens 16 at the incident angle of 0 degree, and when the firstoptical disc (e.g., DVD) 91 is used.

[0150]FIG. 15A shows spherical aberration SA and offense SC against sinecondition at the wavelength of 659 nm; FIG. 15B shows chromaticaberration represented by spherical aberration for wavelengths of 659nm, 654 nm and 664 nm; and FIG. 15C shows astigmatism (DS: sagittal; andDM: meridional). In each graph, the horizontal axis represents amount ofaberration (unit: mm), and the vertical axes of FIGS. 15A and 15Brepresent the numerical apertures, and the vertical axis of FIG. 15crepresents the angle W of view.

[0151] FIGS. 16A-16H show wavefront aberration, under the hypotheticalcondition shown in FIG. 14, when the beam emitted by the first laserdiode 14 a is incident on the objective lens 16 at a predeterminedincident angle (FIGS. 16A-16D: meridional direction; and FIGS. 16E-16H:sagittal direction). In each graph, the horizontal axis represents anentrance pupil, and the vertical axis represents the amount of wavefrontaberration. Further, FIGS. 16A and 16E are graphs when the incidentangle is 0°, FIGS. 16B and 16F are graphs when the incident angle is0.50, FIGS. 16C and 16G are graphs when the incident angle is 0.750 andFIGS. 16D and 16H are graphs when the incident angle is 1.0°.

[0152]FIG. 17 is a graph showing a relationship between the incidentangle of the beam emitted from the first laser diode 14 a with respectto the objective lens 16 and wavefront aberration (rms: root-mean-squarevalue) when the DVD is used. As shown in FIG. 15A, since the objectivelens 16 exhibits the positive coma for the first beam (i.e., for DVD),as the incident angle increases as shown in FIGS. 15B-15D, and FIG. 17,the absolute value of the wavefront aberration increases significantly.

[0153]FIG. 18 schematically shows the objective lens 16 and the secondoptical disc (i.e., CD) 92, under the hypothetical condition where theoptical axis 16X of the objective lens 16 coincides with the normal tothe optical disc 92.

[0154] FIGS. 19A-19C show aberrations of the optical system includingthe objective lens 16 and the protective layer of the optical disc,under the hypothetical condition shown in FIG. 18, when the laser beamemitted by the second laser diode 14 b is incident on the objective lens16 at the incident angle of 0 degree. FIG. 19A shows sphericalaberration SA and offense SC against sine condition at the wavelength of790 nm; FIG. 19B shows chromatic aberration represented by sphericalaberration for wavelengths of 790 nm, 785 nm and 795 nm; and FIG. 19Cshows astigmatism (DS: sagittal; and DM: meridional).

[0155] FIGS. 20A-20H show wavefront aberration, under the hypotheticalcondition shown in FIG. 18, when the beam emitted by the second laserdiode 14 b is incident on the objective lens 16 at a predeterminedincident angle (FIGS. 20A-20D: meridional direction; and FIGS. 20E-20H:sagittal direction). FIGS. 20A and 20E are graphs when the incidentangle is 0°, FIGS. 20B and 20F are graphs when the incident angle is0.5°, FIGS. 20C and 20G are graphs when the incident angle is 0.75° andFIGS. 20D and 20H are graphs when the incident angle is 1.0°.

[0156] Since the objective lens 16 exhibits the negative coma when theCD is used, as shown in FIGS. 20B-20D, and 21, the absolute value of thewavefront aberration increases as the incident angle increases.

[0157]FIG. 21 is a graph showing a relationship between the incidentangle of the beam emitted from the second laser diode 14 b with respectto the objective lens 16 and wavefront aberration (rms: root-mean-squarevalue).

[0158] It is understood by comparing FIGS. 19A, 20B-20D with FIGS. 15Aand 16B-16D, respectively, for the same incident angle, the aberrationsare caused in opposite directions.

[0159] In the above embodiment, the offense SC₁ against sine conditionat the peripheral portion of the common region Rc when the shorterwavelength (659 nm) laser beam is converged on the data recordingsurface of the DVD 91 is 0.0155 mm, the offense SC₂ against sinecondition at the peripheral portion of the common region RC when thelonger wavelength (790 nm) laser beam is converged on the data recordingsurface of the CD 92 is −0.0197 mm. Therefore,SC2/SC1=0.0155/(−0.0197)=−0.787, which satisfies the condition (1).

[0160] With the above-described objective lens 16 and with exemplaryarrangements described below, the coma when the DVD is used and thelaser beam 13 a emitted by the first laser diode 14 a is incident on theobjective lens 16 with an angle of approximately 0.5 degrees formed withrespect to the reference axis 10X is minimized.

[0161] When the normal to the optical disc 2 is inclined with respect tothe reference axis 10X, and the optical axis 16X of the objective lens16X coincides with the reference axis 10X, as in the first and thirdembodiments, the normal to the optical disc is inclined with respect tothe reference axis by 0.24 degrees such that the incident angle withrespect to the optical disc of the laser beam emitted by the first laserdiode is greater than the incident angle with respect to the opticaldisc of the laser beam emitted by the second laser diode. With thisarrangement, in each case where the DVD and CD are used, the coma causedby the laser beam impinging on the objective lens at a certain incidentangle can be cancelled by the coma caused by the optical disc beinginclined.

[0162]FIG. 22 shows a relationship between the wavefront aberration (rmsrepresentation) and the angle formed between the laser beam 13 aincident on the objective lens and the reference axis when the DVD isused, and the normal to the optical axis is inclined at 0.24 degreeswith respect to the reference axis. As understood from FIG. 22, thewavefront aberration is the lowest at the angle of 0.5 degrees, and thewavefront aberration is 0.020λ or lower, which is an allowable level,within a range of −0.3 to 0.3 degrees with respect 0.5 degrees.

[0163]FIG. 23 shows a relationship between the wavefront aberration (rmsrepresentation) and the angle formed between the laser beam 13 bincident on the objective lens and the reference axis when the CD isused, and the normal to the optical axis is inclined at 0.24 degreeswith respect to the reference axis. As understood from FIG. 23, thewavefront aberration is the lowest at the angle of −0.74 degrees, andthe wavefront aberration is 0.020λ or lower, which is an allowablelevel, within a range of −0.35 to 0.35 degrees with respect to −0.74degrees.

[0164] When the optical axis of the objective lens is inclined withrespect to the reference axis, and the normal to the optical disccoincides with the reference axis, as in the second embodiment, theoptical axis is inclined with respect to the reference axis by −0.15degrees such that the incident angle of the laser beam emitted by thefirst laser diode with respect to the objective lens is smaller than theincident angle of the laser beam emitted by the second laser diode withrespect to the objective lens. With this configuration, in each casewhere the DVD and CD are used, the coma caused by the laser beamimpinging on the objective lens at a certain incident angle can becancelled by the coma caused by the objective lens being inclined withrespect to the reference axis.

[0165]FIG. 24 shows a relationship between the wavefront aberration (rmsrepresentation) and the angle formed between the laser beam 13 aincident on the objective lens and the reference axis when the DVD isused, and the optical axis of the objective lens is inclined at −0.15degrees with respect to the reference axis. As understood from FIG. 24,the wavefront aberration is the lowest at the angle of 0.5 degrees, andthe wavefront aberration is 0.020λ or lower, which is an allowablelevel, within a range of approximately −0.5 to 0.4 degrees with respectto 0.5 degrees.

[0166]FIG. 25 shows a relationship between the wavefront aberration (rmsrepresentation) and the angle formed between the laser beam 13 bincident on the objective lens and the reference axis when the CD isused, and the optical axis of the objective lens is inclined at −0.15degrees with respect to the reference axis. As understood from FIG. 25,the wavefront aberration is the lowest at the angle of −0.4 degrees, andthe wavefront aberration is 0.020λ or lower, which is an allowablelevel, within a range of approximately −0.5 to 0.5 degrees with respectto −0.4 degrees.

[0167] As described above, according to the embodiments, the coma causedby the laser beams being incident on the objective lens at an angle withrespect to the reference axis can be cancelled by inclining the opticalaxis of the objective lens relative to the normal to the optical disc.Therefore, when a laser module including two laser diodes incorporatedin a single package is used, aberrations can be well suppressed andappropriate beam spots can be formed on any one of various types ofoptical discs.

[0168] The present disclosure relates to the subject matter contained inJapanese Patent Application No. 2000-112114, filed on Apr. 13, 2000,which is expressly incorporated herein by reference in its entirety.

What is claimed is:
 1. An optical disc drive capable ofrecording/reproducing data to/from an optical disc, said optical discbeing either one of a first disc and a second disc, a protective layerof said first disc being thinner that that of said second disc,comprising: a first laser diode that emits a first laser beam having afirst wavelength; a second laser diode that emits a second laser beamhaving a second wavelength, said second wavelength being longer thansaid first wavelength; an objective lens that converges the first laserbeam on said first disc, and the second laser beam on said second disc;and a driving unit that holds and rotates said optical disc, an opticalaxis of said objective lens being inclined relative to a normal to saidoptical disc, a beam emitting point of said first laser diode beinglocated at a first position, coma, which is caused when the first laserbeam is converged on a data recording surface of said first disc, beingminimized when said first laser beam is emitted from said firstposition, and a beam emitting point of said second laser diode beinglocated at a second position which is different from said firstposition, wherein coma, which is caused when the second laser beam isconverged on a data recording surface of said second disc, is minimizedwhen said second laser beam is emitted from said second position.
 2. Theoptical disc drive according to claim 1 , wherein said objective lens isconfigured such that coma is minimized for a hypothetical disc under ahypothetical condition, the hypothetical disc being an optical dischaving a protective layer whose thickness is intermediate between thatof said first disc and that of said second disc, said hypotheticalcondition being a condition where the optical axis of said objectivelens coincides with the normal to said optical disc.
 3. The optical discdrive according to claim 2 , wherein a first region is defined on saidobjective lens, said first region providing a numerical apertureappropriate for converging said second laser beam on said second disc,wherein said objective lens is configured to satisfy the followingcondition under a hypothetical condition where the optical axis of saidobjective lens coincides with the normal to said optical disc: −4.0<SC ₁/SC ₂<−0.25, wherein, SC₁ represents an offense SC against sinecondition at the peripheral portion of said first region when the saidfirst laser beam is converged on said first disc, wherein SC₂ representsan offense SC against sine condition at the peripheral portion of saidfirst region when said second laser beam is converged on the seconddisc, the offense SC against the sine condition being defined by theformula below: SC=nH ₁/(n′ sinU′)−f(1−m) wherein, n represents arefractive index of the beam incident side medium, n′ represents arefractive index of the beam emerging side medium, U′ represents anangle of the emerging beam with respect to the optical axis, mrepresents a paraxial magnification, H₁ represents a ray height on aprincipal plane, and f represents a focal length.
 4. The optical discdrive according to claim 1 , said optical axis of said objective lensand said normal to said optical disc being included in a referenceplane, said first position and said second position being located onopposite sides with respect to a reference axis, wherein said referenceaxis is an optical axis of said objective lens under a hypotheticalcondition where said optical axis of said objective lens and said normalto said optical disc coincide with each other, and wherein saidreference plane is a plane including said reference axis and said firstand second positions.
 5. The optical disc drive according to claim 4 ,wherein said first position and said second position are arranged suchthat, by arranging the optical axis of said objective lens to beinclined with respect to the normal to said optical disc, said firstlaser beam is converged on a side where a distance between saidobjective lens and said optical disc increases, and said second laserbeam is converged on a side where a distance between said objective lensand said optical disc decreases.
 6. The optical disc drive according toclaim 5 , wherein said objective lens is configured such that coma isminimized for a hypothetical disc under a hypothetical condition, thehypothetical disc being an optical disc having a protective layer whosethickness is intermediate between that of said first disc and that ofsaid second disc, said hypothetical condition being a condition wherethe optical axis of said objective lens coincides with the normal tosaid optical disc.
 7. The optical disc drive according to claim 6 ,wherein a first region is defined on said objective lens, said firstregion providing a numerical aperture appropriate for converging saidsecond laser beam on said second disc, wherein said objective lenssatisfies the following condition under the hypothetical condition wherethe optical axis of said objective lens coincides with the normal tosaid optical disc: −4.0<SC ₁ /SC ₂<−0.25, wherein, SC₁ represents anoffense SC against sine condition at the peripheral portion of saidfirst region when the said first laser beam is converged on said firstdisc, wherein SC₂ represents an offense SC against sine condition at theperipheral portion of said first region when said second laser beam isconverged on the second disc, the offense SC against the sine conditionbeing defined by the formula below: SC=nH ₁/(n′ sinU′)−f(1−m) wherein, nrepresents a refractive index on the beam incident side medium, n′represents a refractive index on the beam emerging side medium, U′represents an angle of the emerging beam with respect to the opticalaxis, m represents a paraxial magnification, H₁ represents a ray heighton a principal plane, and f represents a focal length.
 8. The opticaldisc drive according to claim 5 , further comprising a fine movementmechanism for driving said objective lens to move for focusing andtracking, said reference axis being inclined with respect to the normalto said optical disc, said objective lens being held by said finemovement mechanism such that said optical axis coincides with saidreference axis.
 9. The optical disc drive according to claim 8 , whereinsaid driving unit holds said optical disc such that the data recordingsurface of said optical disc extends in parallel with a bottom surfaceof a case of said optical disc drive.
 10. The optical disc driveaccording to claim 9 , further comprising a mirror member that reflectsthe laser beam emitted by each of said first laser diode and said secondlaser diode to impinge on said objective lens, said mirror member bendssaid reference axis parallel to the data recording surface of saidoptical disc in the optical disc side.
 11. The optical disc driveaccording to claim 10 , further comprising a rough movement mechanismthat drives said objective lens in a direction parallel to the datarecording surface of said optical disc for seek.
 12. The optical discdrive according to claim 8 , wherein said driving unit holds saidoptical disc by inclining the data recording surface with respect to abottom surface of a case of said optical disc drive so that saidreference axis is perpendicular to a bottom surface of a case of saidoptical disc drive.
 13. The optical disc drive according to claim 12 ,further comprising a mirror member that reflects the laser beam emittedby each of said first laser diode and said second laser diode to impingeon said objective lens, said mirror member bends said reference axisparallel to the data recording surface of said optical disc in theoptical disc side.
 14. The optical disc drive according to claim 13 ,further comprising a rough movement mechanism that drives said objectivelens in a direction parallel to the data recording surface of saidoptical disc for seek.
 15. The optical disc drive according to claim 5 ,further comprising a fine movement mechanism for driving said objectivelens to move for focusing and tracking, said reference axis coincidingwith the normal to said optical disc, said objective lens being held bysaid fine movement mechanism such that said optical axis is inclinedwith respect to said reference axis.
 16. The optical disc driveaccording to claim 15 , further comprising a mirror member that reflectsthe laser beam emitted by each of said first laser diode and said secondlaser diode to impinge on said objective lens, said mirror member bendssaid reference axis parallel to the data recording surface of saidoptical disc in the optical disc side.
 17. The optical disc driveaccording to claim 16 , further comprising a rough movement mechanismthat drives said objective lens in a direction parallel to the datarecording surface of said optical disc for seek.
 18. The optical discdrive according to claim 15 , wherein said driving unit holds saidoptical disc such that the data recording surface of said optical discextends in parallel with a bottom surface of a case of said optical discdrive.
 19. The optical disc drive according to claim 18 , furthercomprising a mirror member that reflects the laser beam emitted by eachof said first laser diode and said second laser diode to impinge on saidobjective lens, said mirror member bends said reference axis parallel tothe data recording surface of said optical disc in the optical discside.
 20. The optical disc drive according to claim 15 , furthercomprising a rough movement mechanism that drives said objective lens ina direction parallel to the data recording surface of said optical discfor seek.